The primary pharmacological treatment for depression over the past several decades has been drugs that inhibit synaptic reuptake of monoamine neurotransmitters. Although the importance of monoamine neurotransmission in antidepressant efficacy cannot be discounted, recent evidence indicates other neurotransmitter systems certainly play a role in the mechanism of action of antidepressants. Furthermore, the limitations of current antidepressant treatments, including a large group of non-responders, necessitate the development of novel compounds to treat depression. A growing body of evidence suggests that cholinergic systems may be potential targets for the development of novel antidepressant compounds, and in particular, that excessive activation of cholinergic systems may contribute to the pathophysiology of depression. Studies at the cellular, physiological and behavioral levels have shown that a wide range of antidepressants, including tricyclics, selective serotonin (5HT) reuptake inhibitors, and atypical antidepressants, all act as non-competitive antagonists of nicotinic acetylcholine receptors. More recently, clinical trials have shown that the nicotinic antagonist mecamylamine has antidepressant effects when added to a selective 5HT reuptake inhibitor (SSRI) in human depressed patients non-responsive to the SSRI alone. In the last funding period we showed that interfering with endogenous ACh signaling using both nicotinic antagonists and low efficacy partial agonists of high affinity nAChRs had antidepressant-like effects in mice. We have also found that human depressed subjects show decreased occupancy of high affinity nAChRs with no change in nAChR number, suggesting that increased ACh levels may contribute to human depression. We have hypothesized that antagonism of high affinity neuronal nAChRs is an important component of the therapeutic mechanism of action of classical antidepressant compounds, and further, that nicotinic receptor antagonists may be novel therapeutic agents that could be useful in patients who are not responsive to current pharmacological treatments. Our current hypothesis based on data obtained in the last funding period is that blockade of ACh signaling in the basolateral amygdala along with activity of 5HT-1A receptors in the hippocampus mediate the antidepressant- like effects of nicotinic compounds. We propose the to follow up on this hypothesis and to investigate further the molecular and neuronal mechanisms underlying the antidepressant-like effect of nicotinic drugs by determining whether the antidepressant-like effects of nicotinic antagonists and partial agonists depend on nAChR function in specific neuronal subtypes in the amygdala, identifying pre- and post-synaptic 5HT receptor subtypes necessary for nicotinic-mediated antidepressant effects and determining whether calcineurin activity is essential for the antidepressant-like effects of nicotinic compounds. PUBLIC HEALTH RELEVANCE: Up to 50% of patients with depression are non-responsive to existing antidepressant therapies so it is essential that new medications are developed to treat this crippling psychiatric illness. Emerging reports show that limiting the activity of nicotine receptors in the brain can result in an antidepressant response in patients who were not responsive to a classical antidepressant like Prozac and we have found that nicotine receptor blockers are antidepressant-like in mouse models of antidepressant effects. We propose to identify the brain regions and molecular changes that are responsible for this effect to enlarge our understanding of the brain circuits that are dysfunctional in patients with depression and to find new ways to treat patients who do not respond to existing treatments.